1. Field of the Invention
The present invention relates to a tone correction circuit, and more particularly to a tone correction circuit which can provide an optimum tone for a scene for e.g. fuzzy control.
2. Description of the Related Art
One example of previously known circuits for correcting the tone of a luminance signal is disclosed in JP-A-143158.
FIG. 9 is a circuit diagram of such a conventional circuit for correcting the tone of a luminance signal. FIG. 10 is a graph showing the characteristic of this circuit. In FIG. 9, 101 denotes a constant voltage VA; 108 denotes a constant voltage VB; 124 denotes a luminance signal; 115 denotes a constant voltage; 120 denotes a control voltage; 102, 104, 109, 11, 121, 122 denote NPN transistors, respectively; 106, 107, 113, 114, 116 and 119 denote PNP transistors, respectively; and 103, 110, 117, 118 and 123 denote resistors, respectively.
In the operation of the circuit thus constructed, when the luminance signal voltage exceeds the constant voltage VA, the current which is obtained by dividing the difference between the luminance signal voltage and VA by the resistance of the resistor 103 flows into the NPN transistor 104. Also, when the luminance voltage exceeds the constant voltage VB, the current which is obtained by dividing the difference between the luminance signal and VB by the resistance of the resistor 110 flows into the NPN transistor 111. The currents flowing through the NPN transistors 104 and 111 flow into a differential amplifier composed of the resistor 117, the PNP transistor 116, the resistor 118 and the PNP transistor 119 through current mirrors composed of the PNP transistors 116 and 119, and the PNP transistors 113 and 114, respectively. These currents flow into or out from the resistor 123 with the aid of the control voltage 120 and a current mirror composed of the NPN transistors 121 and 122. Addition of the voltage across the resistor 123 to the luminance signal can provide a linear characteristic bent upward or downward when compared with the characteristic having the ratio of an input to an output of 1:1.
However, the above circuit configuration has the following defect. The linear characteristic bent upward or downward from the line of ratio of 1:1 does not return to the original straight line. If it is desired that different gradients are given to the signal components with lower luminance (VA or lower) and higher luminance (VB or higher), a desired input/output characteristic cannot be obtained. Therefore, optimum control cannot be made for the signals differentially distributed over a high luminance range and a low luminance range.